Ask a science-savvy person what comes to mind when faced with the term “prion,” and the answer will likely be “mad cow disease.” The malady, known in scientific circles as bovine spongiform encephalopathy, famously broke out in the U.K. during the 1980s, infecting at least 180,000 cattle and leading to the slaughter of millions of others because of its infectivity.
Unlike many transmissible diseases, mad cow is not caused by a bacterium or a virus. Rather, the infectious agent is a prion, a misfolded protein that can wreak havoc in the brain. After appearing inside the body, a prion slowly recruits similar proteins to misfold, clump, and clog the nerve-cell network.
Prior to the 1980s, scientists found that prions could pass from person to person under unusual circumstances, such as implantation with infected tissue. But when researchers later discovered that the proteins could cause variant Creutzfeldt-Jakob disease in humans who ate infected beef, prions cemented their reputation as bad actors.
So when a scientist goes on record saying that the signature protein of Alzheimer’s disease, amyloid-β, behaves like a prion and therefore has the potential to be infectious, people take notice—and get a little worried.
That’s exactly what happened late last year after Claudio Soto, a neurologist at the University of Texas Medical School, in Houston, published a study demonstrating that “seed” particles of misfolded amyloid-β from humans can cause damage in the brains of mice (Mol. Psychiatry, DOI: 10.1038/mp.2011.120). When Soto and his group took brain tissue from a deceased Alzheimer’s patient and injected it into the rodents’ brains, they observed amyloid clumps accumulate and spread.
After the paper’s publication, media outlets such as Fox News featured stories with sensational headlines like, “Can You ‘Catch’ Alzheimer’s Disease?”
Soto’s findings caused a stir, even though they were not the first to show similarities between conventional misfolded neurodegenerative proteins and prions. Over the past decade, laboratory evidence has been mounting that both amyloid-β and tau protein, another macromolecule that misfolds and tangles up in the brains of Alzheimer’s patients, behave like prions. Researchers have also seen similar mechanisms of seeding, recruitment, and spreading for α-synuclein, the main protein player involved in Parkinson’s disease.
But the question is, if amyloid-β, tau, and α-synuclein act like prions in the lab, can these misfolded proteins pass from person to person, too?
All of the neuroscientists C&EN interviewed agree that these neurodegenerative proteins are transmissible in the laboratory. But injecting animals with brain tissue is a far cry from what goes on in the real world. No firm evidence exists that the diseases caused by misfolded amyloid-β and its kin are infectious outside the lab, experts say.
On the other hand, few are willing to rule out the possibility that Alzheimer’s might be transmitted from person to person. Like prion diseases, neuroscientists say, Alzheimer’s and other neurodegenerative diseases would not be spread via casual contact, but perhaps via other routes such as blood transfusion or neurosurgery. Still, some in the research community object to labeling amyloid-β and other misfolded neurodegenerative proteins as prions, and thereby scaring the public, until more is known.
The idea that misfolded proteins involved in neurodegeneration, such as amyloid-β and tau, might be transmissible was born in the 1970s from studies of prions. But it was famously revisited in the early 1990s, says Virginia M.-Y. Lee, a pathologist at the University of Pennsylvania Perelman School of Medicine. That’s when Heiko Braak, an anatomist now at the University of Ulm, in Germany, studied the pattern of protein aggregates distributed in the brains of Alzheimer’s patients. Looking at a multitude of autopsied brains, Braak observed that both amyloid plaques and tau tangles moved into specific regions of the brain during different stages of the disease, increasing in density with Alzheimer’s progression.
These “tantalizing” results, Lee says, “suggested that there is a stereotypical manner in which Alzheimer’s pathology spreads” in the brain.
To uncover the mechanism of this patterned spread, though, scientists would first need proper tools—engineered nerve cells and animals—to reproduce and then study the process, Lee says. These tools became available over time.
In 2000, a team led by Lary C. Walker, a neuroscientist now at Emory University, possessed one of the first mice genetically engineered to produce and accumulate in their brains the amyloid-β plaques associated with human Alzheimer’s. “Because the idea had been floating around that Alzheimer’s and other diseases might have some similarities to prion diseases, we decided to test the theory out,” Walker says.
The team injected brain tissue from Alzheimer’s patients into the brains of three-month-old engineered mice. Typically, these mice would not begin developing amyloid plaques until nine months of age. Walker’s team showed that the injections accelerate disease progression: Five months afterward, the rodents already carried the brain plaques (J. Neurosci.2000,20, 3606).
Fast-forward to 2011, when Soto carried out similar experiments with a different type of mouse. Soto’s rodents were also engineered to express the protein building blocks necessary to make human amyloid-β, but the mice were designed not to accumulate plaques during their normal two- to three-year life spans.
About 280 days after injecting Alzhei-mer’s brain tissue, however, the research team observed the protein aggregates in the rodents’ brains. About 580 days afterward, the brains of nearly 100% of the mice were riddled with amyloid plaques.
Walker’s studies were seminal, Soto says. “But we wanted to start with an animal that is ‘normal,’ one that would never develop the disease unless infected,” he says. That’s more akin to what happens in prion diseases, he adds.
Similar experiments have also been carried out for tau protein and α-synuclein. In fact, Penn’s Lee has been instrumental in demonstrating that α-synuclein behaves like a prion with respect to its transmissibility. Her group added synthetic α-synuclein fibrils to mouse nerve cells carrying a normal level of native α-synuclein. The researchers observed those fibril seeds gain access to the cells—although the entrance mechanism is not yet known, she says—and then spread through the neurons. As the fibrils spread, they formed aggregates usually found in the brains of Parkinson’s patients, called Lewy bodies (Neuron, DOI: 10.1016/j.neuron.2011.08.033).
“Once you have a seed of misfolded species inside the cell,” Lee says, “it can corrupt the native protein inside, causing it to adopt the bad conformation.”
Lee and her group have also shown that this process can happen in a mouse’s brain seeded with synthetic α-synuclein fibrils. About 100 days after the researchers injected the brains of two- to three-month-old mice with misfolded synthetic fibrils, Lewy bodies started developing (J. Exp. Med., DOI: 10.1084/jem.20112457). The genetically engineered mice they used typically accumulate aggregates no earlier than eight months of age.
Demonstrating protein transmission with synthetic seeds is important, Lee says, because when the fibrils are derived from brain tissue “you can never be certain that there aren’t other cofactors in there that work together to cause spreading.” In other words, “the best way to prove that the fibrils are wholly responsible for the spreading pathology,” she adds, “is to inject synthetic versions by themselves.”
Researchers hadn’t been able to get experiments with synthetic amyloid-β to work, and thus prove its seed status in Alzheimer’s, until two weeks ago. That’s when a team led by Kurt Giles and Stanley B. Prusiner of the University of California, San Francisco, published an article in Proceedings of the National Academy of Sciences USA (DOI: 10.1073/pnas.1206555109) that did just that: The researchers successfully accelerated the formation of amyloid plaques in the brains of mice by injecting the rodents’ brains with misfolded synthetic amyloid-β particles.
Asked how he and Prusiner, a 1997 Nobel Prize winner for his research on prions, succeeded where others had failed, Giles says others might not have used properly misfolded amyloid-β seeds or might not have waited long enough for plaques to start forming before looking at rodents’ brains.
To address the latter problem, Giles and Prusiner used a technique called bioluminescence imaging. They genetically engineered mice so that the rodents’ brains fluoresce when amyloid plaques start to build up. Giles explains that, with the imaging technique, the team can see the fluorescence through the rodents’ skulls. “Then we can say, ‘Right, we know that we have disease progression, so now’s the time to take the brains out and analyze them,’ ” he adds.
Taken together, these results point to amyloid-β and other neurodegenerative proteins as behaving like prions, says Neil R. Cashman, a neurologist at the University of British Columbia, in Vancouver. “It’s becoming a widely accepted idea,” he adds. “But it’s also opening a Pandora’s box.”
Only one in a million people in the U.S. die of a prion disease each year, according to the Centers for Disease Control & Prevention. Those diseases are never transmitted from person to person via casual, or even intimate, contact, Cashman says. But people have contracted prion diseases via tissue implants, tainted neurosurgical instruments, or blood transfusions.
So far, there’s been no evidence of infection via any of those routes for neurodegenerative proteins such as amyloid-β, but it’s something to keep an eye on, Cashman adds. “We have all been of the opinion that there is no public health risk” in the spreading of these misfolded proteins, he says, but “the recent experimental data make it a valid concern that deserves further research.”
Some infectivity research has already been carried out. In 2009, Mathias Jucker of Tübingen University, in Germany, along with Emory’s Walker, implanted stainless steel wires coated with amyloid-β seed particles into the brains of mice. The wires were meant to simulate the use of steel instruments in neurosurgery.
Plaques formed in the tissue surrounding the wire and, to a lesser extent, elsewhere in the rodents’ brains. Heating the wires to 95 °C before implantation did not prevent the misfolded proteins from spreading, but plasma sterilization did (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0903200106).
UT Houston’s Soto has also been making some noise recently about unpublished work from his group showing that amyloid plaque formation can be triggered in the brains of mice through blood transfusion. Cases of prion diseases transmitted to others by infected blood donors are known, Soto says, “so we wanted to look at whether this process is applicable to neurodegenerative diseases as well.” Although he won’t yet discuss the details, he says “the results seem to be positive” and suggest that amyloid-β seeds can cross the blood-brain barrier.
Knowing that amyloid-β and similar proteins act like prions, researchers are left wondering why no one has recorded a case of the proteins passing from person to person. On the basis of laboratory results, “we all think they should be infectious,” Jucker says, “but there’s no evidence.” He and others are awaiting Soto’s blood transfusion results to be published before further considering the possibility.
Soto and Cashman say evidence for infectivity may be lacking because proper epidemiological studies have just not been performed yet. “Neurodegenerative disease symptoms take a long time to show up,” Soto says. “A person might be exposed one day to a misfolded protein but show signs of the disease several decades afterward. It would be difficult to track.”
According to Cashman, one test of the infectivity hypothesis might be to examine blood transfusions from elderly donors to young recipients. “If the incidence of nongenetic Alzheimer’s increased 30 years after the transfusions,” he says, “then we might get a hint that these diseases are transmissible.”
Another reason there might be a lack of evidence that these neurodegenerative proteins are infectious, Emory’s Walker says, is that their misfolded forms just aren’t as sturdy as prions. “Prions are remarkably resistant to destruction,” he says. “It’s likely that a lot of these other proteopathic seeds are much less so and are perhaps more easily degraded by enzymes in the body.” That’s an idea that will need to be investigated, he adds.
Until researchers know the answers to these questions, though, “should we be scaring people with a possibility that may not be a problem at all?” Walker asks. “That’s the essence of the controversy going on in the field right now.” On the other hand, he adds, “it would be a mistake to ignore something that is a potential health risk.”
In the meantime, some in the field are calling for researchers to be careful about how they discuss their work with the media and public. In a recent article, John Hardy and Tamas Revesz of University College London called on fellow scientists to avoid using the term prion to describe amyloid-β and similar proteins associated with neurodegenerative disease (N. Engl. J. Med., DOI: 10.1056/NEJMcibr1202401).
“We’re in danger of devaluing the word ‘ prion, ’ ” Hardy says. Prions can be highly infectious, particularly among animals. For instance, studies have shown that prions are so robust, they stick around in soil years after flocks of infected sheep roamed on top of it, Hardy says. That situation is hardly comparable with the one for amyloid-β, he argues. Prions and these other proteins may spread by similar mechanisms, Hardy says, but nothing like the devastating mad cow disease transmission of the 1980s has ever been observed for Alzheimer’s. “It’s irresponsible to stir up that fear,” he says.
Despite the concerns this current wave of research has dredged up, Penn’s Lee and others see a silver lining. What these results provide the research community, Lee says, are potential new drug targets at which to block progression of these devastating neurodegenerative diseases. “This really creates new opportunities,” she says, “and we’re very excited about it.”